1. Field of the Invention
This invention, in general, relates to a method and apparatus for locating hidden ferromagnetic objects. More particularly, this invention relates to a method and apparatus for locating underground ferromagnetic objects without excavation. More particularly yet, this invention relates to a method and apparatus for locating underground cast iron pipes and cast iron pipe joints employed in underground fluid distribution systems, such as natural gas pipelines.
2. Description of Related Art
Many water and gas utilities have distribution pipelines constructed of cast iron. With time, problems generally develop which cause fluid leakage. For example, in the case of gas utilities, seepage of gas from the bell and spigot joints connecting the pipe sections can occur. Technologies have been developed to seal these joints; however, to make the repairs requires excavation of the pipe. Thus, the ability to accurately locate the cast iron pipe joints is essential to maintaining the costs of excavation as low as possible.
New techniques enable the use of small diameter holes drilled from above ground to access the cast iron pipe joint. It will, thus, be apparent that reducing the number of holes required to be drilled in order to locate the cast iron pipe joint is a desirable objective. Accordingly, a device that is capable of accurately locating the joints may reduce or eliminate the boring of holes that miss the cast iron pipe joint target.
Many technologies, both active and passive, have been used in the past to determine the location of underground objects such as utility pipelines, power lines and the like. Among these technologies are nuclear, acoustic, gravitational, magnetic and electromagnetic methods and systems. Within the electromagnetic area of technology, infrared, microwave, low-frequency magnetic and eddy current devices have been employed.
A wide variety of metal detectors have also been used to locate buried objects, for geophysical exploration and for other applications as well. These devices typically operate to create a near field of continuous wave electromagnetic forces about a central inductive coil. Detection of a buried metallic object is evidenced by an impedance change. However, these devices are also sensitive to variations in the soil, resulting in numerous false positive signals. Many continuous electromagnetic wave type detectors are typically constructed of a transmit coil mutually coupled with a receiver coil. However, the requirement of mutually coupled coils inhibits the use and application of these types of devices. Because the geometry of the coils is critical for operation, the device must be constructed of rigid members to maintain precise relative coil placement. These types of devices are very sensitive to slight jarring or impact, which may cause coil movement, rendering the device inaccurate.
U.S. Pat. No. 5,617,031 to Tuttle teaches a buried pipe detection device based upon the principle of detecting differences in the dielectric/conductive properties of the ground in the vicinity of the pipe. The electrical properties of the ground directly over the buried pipe are said to differ from those in an area immediately adjacent to the pipe. The device employs a capacitor plate array that injects a low frequency sinusoidal signal into the ground and samples the injected signal from two spatially separated locations of the array.
U.S. Pat. No. 5,640,092 to Motazed et al. teaches an electromagnetic pipe mapper for determining the location and depth of buried pipes. The device includes a target signal transmitter for inducing an electromagnetic signal in the buried object, a position reference transmitter for transmitting a signal for positioning purposes, a sensor unit for detecting changes in the magnetic flux of the buried object due to the induced electromagnetic signal and the positioning signal from the position reference transmitter, and a central unit for processing and displaying a map of the buried object.
Although, as evidenced by the above referenced U.S. patents, devices exist for locating buried pipe, none of these known devices is able to detect buried pipe joints or other buried pipeline features.